This section provides background information related to the present disclosure which is not necessarily prior art.
Because carbon dioxide (R744) has a low global warming potential (GWP) of only 1 and no ozone-depleting potential at all (ODP of zero), it makes an excellent environmentally friendly refrigerant as compared to hydrofluorocarbons, hydrofluoroolefins, and other less environmentally sound refrigerants. However, the pressures required to liquefy carbon dioxide prove to be too high for use in conventional heating and cooling systems. To avoid high pressures in a refrigeration cycle, carbon dioxide can be used along with a so-called co-fluid or mixture of co-fluids.
In operation of an HVAC&R system using carbon dioxide and a co-fluid, carbon dioxide refrigerant is absorbed into and desorbed out of the co-fluid. For example, carbon dioxide is absorbed and the pressure lowered during compression and flow through a condenser or absorber. Subsequent flow through an expansion device and evaporator requires a desirable release (desorption) of a portion of the carbon dioxide refrigerant.
It has generally been observed that rates of absorption tend to be faster than rates of desorption in co-fluid systems using carbon dioxide as refrigerant. This rate inequality can potentially lead to problems in operating the heating and cooling system. There may not be enough time for proper heat flow to the evaporator needed for cooling. And there could be an accumulation of carbon dioxide in the co-fluid due to the rate difference, causing the system to be inefficient or even inoperable. There is a continuing need for co-fluids that provide a higher rate of desorption to improve operation in cooling systems.